The nuclear low molecular weight RNA species of mammalian cells have been known for over a decade but their function remains unknown. RNA species D (Ul), which inhibits the cell-free translation of mammalian messenger RNA and whose cytoplasmic concentration is high only during mitosis, could be involved in the decrease of the rate of protein synthesis that occurs during mitosis. In addition, it has been postulated recently that D RNA may participate in the process of messenger RNA splicing, since a nucleotide sequence near the 5' end of D RNA exhibits extensive base complementarity to the consensus splice junction sequences in messenger RNA precursors. The 5' ends of RNA species D, C (U2) and A (U3) are capped by a structure that contains 2,2,7-trimethylguanosine (m3-2,2,7G). We have prepared antibodies against m3-2,2,7G. We wish to test the effect of these antibodies on protein synthesis and on messenger RNA maturation, after they have been microinjected into mammalian cells or after they are added to mammalian cell-free extracts.